1. Field of the Invention
The present invention relates to static inverters by which electrical energy in the d.c. form is converted to electrical energy in the a.c. form through non-moving or static means. The present invention falls into the class of inverters in which a d.c. source is used to produce current through a pair of alternately switched semiconductor devices connected in series with the primary windings of a power transformer and producing an a.c. output in the transformer secondary.
2. Description of the Prior Art
Static inverters of the class empolying a d.c. source, a pair of switching semiconductor devices, and a transformer are in common use. The circuit exists in either the free running or the driven form. In the free running form, feedback windings provide positive feedback necessary for oscillation and sometimes negative feedback for control purposes. Once started, the inverter, will oscillate with alternate conduction of the power transistors and the reversals in current in the primary windings will produce an alternating voltage in the output winding. In the feedback circuit, the base drive is normally limited by a serially connected resistor. The converter will switch from one direction of conduction to the other when the collector current demand on the output circuit due to magnetizing current plus reflected load current exceeds the collector current that can be sustained by the resistance limited base drive current. For a fixed load, the switching occurs when the core saturates, the primary impedance falls, and the magnetizing current starts to rise. The amount of core saturation will change under variable load conditions since the ratio of magnetizing current to reflected load current changes. When power transistors are used in high power application, the transistors must have high sustained collector to emitter voltage ratings as well as a high current handling capacity. The high voltage transistors must be fabricated with high purity semiconductor material to obtain a high voltage rating and this results in transistors that exhibit long storage times. Shorter times can be achieved by doping the active region but this reduces the voltage rating. At the moment of commutation, the storage time delays the actual transistor turn off from that commanded by the base drive. The result is that very large instantaneous current spikes can be generated that will result in transistor failure if the core is heavily saturated. When external drive is used to prevent magnetic saturation other problems arise. The storage time will result in conduction overlap if complementary drive signals are applied to the two switches. At start up, the tolerable drive period is reduced from that of the running state because the linear core material starts from a remnant state that is less than the .+-. peak flux density achieved in the running state. Thus, the drive period for the first half cycle must be reduced from the running state drive period.
An approach related to that of the present invention is that of U.S. Pat. No. 3,914,680 by Joseph P. Hesler and Samuel M. Korzekwa, entitled "Static Inverter". In that application, a single aperture was introduced into a magnetic core to divide the core into two branches and regenerative and degenerative windings were introduced coupled to one or the other branch and both switching devices were controlled from the same aperture. The inverters there depicted, while having essentially equal peak loads to those herein disclosed, were quite subject to negative collector currents, limiting their ability to operate at minimum loads. The circuits also were unoptimized in respect to power dissipation in the control circuitry and/or in the number of components.